what's with topozone accuracy in northern Delaware?

[+edscott]

Have yet to find a cache in Northern Delaware using Topozpone data.

 

I don't own a GPS and have never had huge problems finding a cache using topozone maps of other areas by going to the red "X" and looking around.

 

I have tried "Ibnub" near Newark Delaware and now have data that says the cache is about 600 feet North of the spot marked on the map. I have downloaded maps of two more caches nearby... one in Brandywine Park and one just north of there and the map data is clearly wrong. I am ready to try one near I 95 near Wilmington and wonder if it will be there either. Is there a consistant error that I can calculate, or is it strictly a hit or miss thing in that part of Delaware? Edscott

[+Mopar]

I see by your profile your pretty good at maps and gps-less caching! You probably already know this then, but in case you don't and for any others that might be wondering the same thing, here goes.

There are lots of different map datums (I'll explain datums below), but the only 2 important ones here are WGS84 and NAD27. The current standard is WGS84. GPS receivers all use this one by default. Cache waypoints on geocaching.com are also in WGS84. The problem is, All the topo maps in the US are based on NAD27. These are usually close to each other, but not exact. Since the differences between to 2 are not uniform and vary from area to area, geocaching.com does not make the corrections when it passes info to topozone.com. If you notice there is a note next to the link stating that the coordinates might be off. You might try using lostoutdoors.com instead of topozone, I think they make an attempt to convert WGS84 to NAD27 when displaying waypoints on the maps.

Hope that helps someone!

 

For those wondering just what the heck a "datum" is, here ya go:

quote:

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From maptools.com:

A datum describes the model that was used to match the location of features on the ground to coordinates and locations on the map. Maps all start with some form of survey. Early maps and surveys were carried out by teams of surveyors on the ground using transits and distance measuring "chains". Surveyors start with a handful of locations in "known" positions and use them to locate other features. These methods did not span continents well. Frequently they also did not cross political borders either. The "known points" and their positions are the information that the map datum is based. As space based surveying came into use, a standardized datum based on the center of the earth was developed.

 

Every map that shows a geographic coordinate system such as UTM or Latitude and Longitude with any precision will also list the datum used on the map.

 

The Global Positioning System uses an earth centered datum called the World Geodetic System 1984 or WGS 84. WGS 84 was adopted as a world standard from a datum called the North American Datum of 1983 or NAD 83. For all practical purposes there is no difference between WGS 84 and NAD 83.

 

Most USGS topographic maps are based on an earlier datum called the North American Datum of 1927 or NAD 27. (Some GPS units subdivide this datum into several datums spread over the continent. In the Continental United States use NAD27 CONUS.)

 

In the Continental United States the difference between WGS 84 and NAD 27 can be as much as 200 meters.

 

You should always set your GPS unit's datum to match the datum of the map you are using.

 

On a USGS topographic map the datum information is in the fine print at the bottom left of the map. The datum will always be NAD 27. There may be information on how many meters to shift a position to convert it to NAD 83. Think of this as the error that will be introduced if you leave your GPS unit set to WGS 84. A dashed cross in the SW and NE corners of the map gives a visual indication of the difference between the two datums.

 

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Tae-Kwon-Leap is not a path to a door, but a road leading forever towards the horizon.

[+edscott]

yes, I think I am correcting the datum issue through Jeeep.com before I go to topozone... at least it has worked with the others I have found and set. .. but for some reason there seems to be another factor for the two or three quads in Northern Delaware. Perhaps jeeep or topozone has a programming glitch?? I'll see how the other sources listed work... ed

[+Mopar]

I kinda figured you would be on the ball, looking at your profile, I can tell you know what end of a map is north, lol.

I did think maybe there where others with your same type of question though, and i do't recall it ever coming up in the forums, so it deserved a complete answer.

Let us know if you figur eout where the error is.

Chris

 

Tae-Kwon-Leap is not a path to a door, but a road leading forever towards the horizon.

[+edscott]

Thank you... now busy reprinting all my Delaware files...

[+edscott]

Have rechecked four in the Wilmington - Newark area using lostoutdoors and the topozone error is 225-250 meters at 200 degrees. ...that is topozone puts you 235+/- meters... over 750 feet... ssw of the cache.

[+Poindexter]

"ibnub" is in the Newark West quad. The datum for that quad as well as for other northern Deleware quads is NAD83. Most U.S. quads are in NAD27 but NOT all. Here's the quad info for Newark West.

 

24k Quadrangle Information You Clicked on..

Longitude: -75.81 Latitude: 39.68

State: Maryland County: Cecil

Quadrangle Name: NEWARK WEST

Size: 7.5 x 7.5 - Minute, 1:24,000

Map Source Date: 1993/07/01

 

DRG Creation Date: 2002/05/03

DRG Coordinate System: UTM

DRG Datum: NAD 83

 

Poindexter

[+edscott]

Ok!!! that completes the solving of the mystery ... ed